The properties of polymers can vary over a wide range and are a consequence of the structure of the polymer and the interactions of the individual polymer building blocks and a broad range of macromolecular parameters. The particular state of a distinct polymeric material at any instance depends on the temperature and deformation rate of the material, as well as other factors. The properties of a specific linear polymer vary from a solid (glassy or crystalline) state at lower temperatures to a liquid-like state at high temperatures. The modulus may vary from higher than 109 Pa for the glassy state to lower than 104 Pa for the melt state. By increasing the deformation rate at a constant temperature, the properties can change within the same range but in the opposite direction i.e. from liquid-like to the solid-like behavior at low and high deformation rates, respectively.
An intermediate rubbery state, where rubber-elastic properties are developed, is very specific to polymers. In amorphous polymers, the elastic properties result from entanglement of polymer chains, if the chains are sufficiently long. This rubbery state may be observed in either the modulus vs. temperature, or in the modulus vs. deformation rate curves, as a rubbery plateau. The rubbery state does not appear for a polymer having a molecular weight below the critical molecular weight for entanglements (Me). The length of the plateau is very much dependent on the length of the molecular chains or the molecular mass of the polymer. The height of the plateau is only weakly dependent on the type of the polymer and typically assumes a value in the range between 105 and 106 Pa. Typical experimental values for the shear modulus of this rubbery plateau for some commodity polymers are given in Table 1. It is believed that the modulus of the rubbery plateau is proportional to the number of entanglements, as illustrated in Table I, it decreases with increasing molecular weight between entanglements (Mc).
TABLE 1Values of the rubbery shear modulus of some polymers(from D. W. van Krevelen, Properties of Polymers,Elsevier, Amsterdam 1997)Molecular Weight betweenentanglementsPolymer103 g/molTg [K]Gr 105 PaPolypropylene (am)72585Poly(vinyl chloride)6.23564Polyisobutylene161983Poly(n-butyl acetate)282261.5Poly(vinyl acetate)253011.3Polystyrene353751.3
The typical mechanical behavior of an amorphous polymer under small dynamic deformation is illustrated in FIG. 1 showing dependencies of components (real G′ and imaginary G″) of the complex shear modulus, G, on temperature (FIG. 1a) and deformation frequency (FIG. 1b). The transitions between the different states, glass, rubber and melt, may be influenced by the molecular dynamics. The material transforms from a glass to a rubber due to the local segmental mobility of polymer chains. It remains in the rubbery state as long as the deformation period is shorter than the time necessary to disentangle a polymer chain from an entangled network of other chains. The time necessary to disentangle a polymer chain corresponds to the rotational relaxation times of linear polymer chains in the melt [T. Pakula, S Geyier, T Ediing and D. Boese, Rheologica Acta 35, 631(1996)]. At deformation rates that are slow in comparison with the relaxation time, the system flows and enters the melt state.
The rubbery state can, however, be fixed by chemical or physical cross-linking of the polymer chains. The characteristic rubbery properties, then, remain over a very broad range of variation of the temperature and of deformation rate. This behavior results in the formation of a plateau in the shear modulus curves, shown schematically by means of dashed horizontal lines in FIGS. 1a and 1b. 
Crosslinking a polymer results in the formation of a polymer network. There are several ways to obtain a polymer network. Some of them are illustrated two-dimensionally in FIG. 2. Cross-linked polymers (elastomers) of both natural and synthetic origin constitute the oldest class of commercial polymeric materials (rubbers) and are still one of the basic industrial products. Physical characteristics of an elastomer are a low modulus and the capacity for high deformability, when compared with hard solids (e.g. glass or metal). Elastomers have a typical Young's modulus value (at small strains) of the order of 106 Pa, with reversible extensibility reaching 1000%.
The main characteristic property, the shear elastic modulus, G, at small deformations for such rubbery materials, is determined by Equation (1) derived on the basis of fundamental thermodynamic considerations, assuming that the work of deformation corresponds to variation of entropy related to extension of Gaussian chains,G=ρRT/Mc  (1)where ρ is the density of the rubber, R is the gas constant per mole and Mc is the number average molecular weight between cross-links [L. R. G. Treloar in “The Physics of Rubber Elasticity” Clarendon Press, Oxford 1975].
As this relationship indicates the properties of these materials can be modified by variation of cross-link density. Weakly cross-linked rubbers preserve the modulus of the rubbery plateau seen for the melt of linear entangled polymers (Mc remains similar to the order of Me) is higher. The shear elastic modulus may be increased by increasing the cross-link density or as a result of physical cross-linking, such as, crystallization or by microphase separation in block copolymers comprising incompatible segments (thermoplastic elastomers). [Thermoplastic Elastomers, A Comprehensive Review Edited by Legge, N. R.; Holden, G.; and Schroeder H. E. Hanser Publication 1987].
However, it is not easy to reduce the shear elastic modulus below the bulk plateau modulus of a given polymer, but the plateau modulus may be lowered considerably in polymer solutions, [P. -G. de Gennes in “Scaling Concepts in Polymer Physics”, Cornell University, Ithaca 1979; T. Inoue, Y. Yamashita, K. Osaki, Macomolecules 35, 1770, 2002] and soft gels may be obtained by swelling weakly cross-linked systems with a solvent. However, solvent swollen polymers gels are not stable in environments in which the solvent can evaporate or in some instances when external forces are exerted on the gel. In networks of strongly hydrophilic polymers (e.g. hydrogels), shear elastic moduli in the order of 103 Pa can be obtained at low cross-link densities and relatively high degrees of swelling. Higher levels of swelling may result in an increase in shear elastic modulus, because of strong extension of the network chains. [e.g. U. P. Schröder and W. Oppermann, in “Physical properties of polymer gels” Ed.: J. P. Cohen Addad, John Wiley & Sons, Chichester 1996].
Therefore, there is a need for softer materials that do not comprise a solvent. There also exists a need for a bulk polymer network with a Young's modulus less them 105 Pa.